What is a blend file?
The .blend files are the project files of Blender.

They are binary files with the .blend extension as in fileName.blend. Blender saves its data as .blend files and can only open .blend files.

Other 3D files types are not "opened", but imported.

Blend files contain the models, materials, the scene (camera, lights and animation) and the information for rendering and viewing along with the state of the user interface.
By default they don't contain images used as textures unless you pack them. Read: How to pack images into blend file

Also not contained as part of the file are simulation caches or the add-on used to create the scene.